The plant's cell connections called plasmodesmata mediate symplasmic communication processes which play a crucial role in the control of plant development. Several developmental regulators, including a variety of transcription factor proteins and sRNA species, have been shown to move through plasmodesmata in order to regulate gene expression non-cell-autonomously on the transcriptional and posttranscriptional level. The symplasmic exchange of such regulatory molecules is a crucial element in

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... complex molecular networks controlling plant growth and morphogenesis. It is generally accepted that plasmodesmal communication is essential for the coordination of cell-division activity, cell-fate specifi cation, tissue patterning, and organogenesis. Dynamics of the plasmodesmal networks in the developing tissues are supposed to facilitate modulations of the intercellular communication pathways which correlate with the developmental requirements. The symplasmic organization can be modulated to cause morphogenetic switches in response to environmental or endogenous signals. In the present review, we summarize the distinct modes by which structural and functional alterations of the plasmodesmal networks can be achieved, and we discuss possible molecular control mechanisms. Moreover, we will give an overview on the programmed developmental changes in the number, structure, and in the functional state of plasmodesmata which occur in growing plant tissues, including embryos, leaves, roots, and shoots during primary and secondary growth, as well as during the transition to fl owering. Keywords Cambium • Morphogenesis • Plant development • Plasmodesmal deletion • Plasmodesmal fi ssion • Plasmodesmal structure • Plasmodesmal origin • Secondary plasmodesmata • Symplasmic domains • Tissue patterning which belongs to the CLE (CLAVATA3/EMBRYO SURROUNDING REGIONrelated) protein family, undergoes posttranslational modifi cation to form a small signaling peptide. This peptide is secreted into the extracellular space and binds to CLV1, a leucine-rich repeat receptor kinase predominantly expressed in cells of the rib zone. Alternatively, peptide binding to related receptor complexes composed of CLV2 and CORYNE/SUPPRESSOR OF LLP1 2 (CRN; Guo et al. 2010 ) and to RECEPTOR-LIKE PROTEIN KINASE 2 (RPK2; Kinoshita et al. 2010 ) is also possible. To complete the feedback loop, CLV3 perception by the receptors represses the WUS transcription by a mechanism that is not well understood, but may include the downregulation of the activity of the phosphatases POLTERGEIST1 and POLTERGEIST1-LIKE 1 (Guo et al. 2010 ) . Further effectors of the complex control network regulating the WUS expression in the SAM are described in Fig. 2.1 . Similar models have also been presented for the Arabidopsis root apical meristem (RAM; e.g.